1. Field of the Invention
The present invention relates to surgical instruments for laser cardiac ablation procedures. More particularly, the invention relates to an ablation apparatus with a guide member to guide the ablation apparatus in a desired pattern.
2. Description of the Prior Art
A. Atrial Fibrillation
It is known that at least some forms of cardiac arrhythmia are caused by electrical impulses traveling through the cardiac muscle tissue by abnormal routes. In a normal, non-arrhythmic heart, electrical nerve impulses travel in an orderly and well-defined fashion through the sinoatrial node and then through the atrioventricular node in order to create an orderly flow of electrical impulses that lead to contraction in the heart.
In cardiac arrhythmias, cardiac impulses travel along undesirable pathways through the cardiac tissue leading to a rapid heart beat (tachycardia), slow heart beat (bradycardia) or a disorderly heart beat (fibrillation). Atrial fibrillation (AF) is a chaotic heart rhythm of the atrial chambers of the heart. Atrial fibrillation prevents the heart from pumping blood efficiently causing reduced physical activity, stroke, congestive heart failure, cardiomyopathy and death.
B. Maze Procedure—Generally
One technique for treating atrial fibrillation is to surgically create lines in the heart muscle tissue (myocardium) whereby electrical conduction of nerve impulses is blocked or rerouted. This technique for creating lines of electrical blockage is referred to as the Maze procedure.
Initial approaches to performing the Maze procedure involved invasive surgery in which a series of linear incisions are made in the cardiac tissue and then sutured together. The lines of scar tissue that form in the incisions do not conduct electrical impulses and are intended to prevent disorderly contraction of the atrial tissue.
In a typical Maze procedure, up to six non-conductive lines are required. Each of the non-conductive lines is typically several centimeters in length. Once these lines scar and heal, they disrupt electrical pathways that may cause atrial fibrillation. Examples of the Maze procedure and other surgical techniques for treating atrial fibrillation are described in Chiappini, et al., “Cox/Maze III Operation Versus Radiofrequency Ablation for the Surgical Treatment of Atrial Fibrillation: A Comparison Study”, Ann. Thorac. Surg., No. 77, pp. 87-92 (2004) and Cox, “Atrial fibrillation II: Rationale for surgical treatment”, J. Thoracic and Cardiovascular Surg., Vol. 126, No. 6, pp. 1693-1699 (2003).
C. Less Invasive Maze Procedure Technologies
Less invasive ablation techniques have also been utilized to perform the Maze procedure. In such techniques, the surgeon typically drags an a radiofrequency (RF) electrode in a linear fashion along the endocardial (internal) or epicardial (external) surface of the heart to produce a series of lesions using heat to desiccated and ultimately kill cardiac cells. The scaring created by the lesions is ideally contiguous and non-conductive of electrical impulses. For endocardial use, standard ablation catheters or catheters with extended distal electrodes are employed. Epicardially, specially designed handheld probes with a distal electrode for the application of ablating energy are often used.
For the greatest likelihood of success in a Maze procedure, it is particularly important that the lesions created be transmural. A transmural lesion extends through the full wall thickness of the cardiac muscle at the location of the lesion. One factor that limits transmurality of lesions from the epicardium is the cooling effect of blood in and around the heart particularly during ‘off-pump’ procedures during which the heart is beating. This is particularly difficult when radio frequency (RF) energy is employed because it relies exclusively on thermal diffusion to create transmural lesions i.e, flow of heat from higher to lower temperature. The cooling effect of blood on the endocardial surface within the atrium limits attainment of the temperature required to form thermal lesions.
The maximum temperature, at electrode/tissue interface, is also limited to something less than the boiling point of water. Higher temperatures cause boiling of interstitial water creating explosions and subsequent tissue perforations. Perforations of the atrial wall leads to a weakening of the heart structure as well as significant bleeding during surgery that must be controlled.
Additionally, high electrode/tissue temperatures can create burns and adhesion between the probe and the heart tissue. Such adhesions can insulate the probe from the heart tissue blocking the efficient application of energy. These procedures are also a problem for the surgeon and staff who often must stop to clean the tip of the probe.
The efficacy of creating transmural lesions with RF can be enhanced by using a second electrode at the endocardial surface. The endocardial electrode provides a more direct electrical path through cardiac tissue which ‘focuses’ the energy more directly at the target site and secondarily protects the endocardial surface from direct cooling by blood flow in the left atrium. This approach requires access into the left atrium which adds complexity and increases risk to the patient.
The same analysis can also be applied to cryogenic methods which freeze interstitial water causing cellular death. However in this application, the blood warms the tissue at the endocardial surface which again limits the attainment of temperatures required to cause cellular death and create transmural lesions.
A discussion of techniques and technologies for treating atrial fibrillation is set forth in Viola, et al., “The Technology in Use for the Surgical Ablation of Atrial Fibrillation”, Seminars in Thoracic and Cardiovascular Surgery, Vol. 14, No. 3, pp. 198-205 (2002). Viola et al. describe numerous ablation technologies for treating atrial fibrillation with the Maze procedure. These include cryosurgery, microwave energy, radiofrequency energy, and laser ablation.
D. Laser Ablation and the Maze Procedure
The use of lasers in treating atrial fibrillation is desirable because laser energy is first and foremost light which is subsequently converted to heat. Thus, the principles for transmission of light can be used to ‘diffuse’ laser energy in cardiac tissue. At selected wavelengths, light diffusion can be significantly faster and penetrate more deeply than thermal diffusion. To achieve this effect, it is important to understand the spectral characteristics of atrial tissue and select a laser wavelength with high transmissivity, i.e., low absorption. Wavelengths in the near infrared region, 700-1200 nanometers are suitable for achieving such results. Ideally the wavelength would be 790 to 830 or 1020 to 1140 nanometers. As a result, laser ablation is fast and results in narrow lesions. Viola, et al., “The Technology in Use for the Surgical Ablation of Atrial Fibrillation”, Seminars in Thoracic and Cardiovascular Surgery, Vol. 14, No. 3, pp. 201, 204 (2002). However, in the prior art, laser ablation for treating atrial fibrillation has been troublesome.
Viola et al. discuss problems associated with the use of laser energy to treat atrial fibrillation. These concerns are directed to safety and reliability and note that lasers are prone to overheating because of the absence of a self-limiting mechanism. The authors note that over-heating with lasers can lead to crater formation and eventually to perforation, especially when using pin-tip devices. Viola, et al., supra, at p. 203. The authors note that the high power of laser ablation (described as 30 to 80 Watts) results in the laser technique not being widely clinically applied. Id., at p. 201. The mechanical effects resulting from direct heating of the myocardial tissue with laser energy results in cellular explosions caused by shock waves. Viola, et al., supra, at p. 201.
The possibility for perforation of the myocardium with laser energy raises a particular concern for treating atrial fibrillation. The myocardial wall of the atria is quite thin (e.g., about 2 mm in thickness in some locations). A coring of the myocardium by a laser could result in a full wall thickness perforation and resulting leakage of blood.
Viola et al. note the development of a long probe laser that allows diffusion of the laser thermal energy over the long probe tip in a unidirectional fashion. Id., at p. 201. While not mentioning the source of this long probe tip, it is believed by the present inventors to be referring to the atrial fibrillation laser of CardioFocus, Inc., Norton, Mass. (USA) as described in U.S. Patent Application Publication No. 2004/6333A1 in the name of Arnold, et al. (published Jan. 8, 2004) and U.S. Pat. No. 6,579,285 issued to Sinosky. This technology as practiced differs in two ways to that of the present invention. First, and most importantly, it defocuses the coherent laser beam by using reflective particles to scatter the light longitudinally and radially before it enters the tissue. This reduces the longitudinal movement required to produce linear lesions but, by decreasing the coherency of the laser beam before entering cardiac tissue, and negates many of the advantages of light to more deeply penetrate cardiac tissue. Secondly, this technology uses laser light in the 910 to 980 nanometer wavelengths which has a significant water absorption peak compared to 810 and 1064. The higher absorption reduces the penetration of the laser light through cardiac tissue. Reducing energy penetration depths increases the risk (particularly on a beating heart) of creating a lesion that is less than transmural.
E. Conductivity Verification
A further difficulty with creating linear nonconductive lesions is the inability to verify that a truly nonconductive lesion has been produced. If a transmural lesion is not properly formed in accordance with the Maze procedure, the treatment for atrial fibrillation may not be successful. This could require a second surgical procedure. It would be helpful if the surgeon could promptly discern whether a particular linear lesion is truly non-conducting at the time of the original procedure to permit correction at that time. This would enable prompt re-treatment if necessary.
F. Placing and Guiding an Atrial Ablation Tool
The afore-mentioned U.S. patent application Ser. No. 10/975,674 describes formation of a lesion pattern by a surgeon moving the tip of a wand over the heart surface. Use of a tool to guide or control an ablation tool has been suggested. For example, U.S. Pat. No. 6,579,285 (assigned to CardioFocus, Inc.) shows a diffused light fiber tip in a malleable housing. The housing is bent to form a desired shape and placed against the heart. The diffused light fiber tip is moved through the housing in a series of steps to form a lesion. The lesion is formed by stopping the fiber at a location, energizing the motionless fiber to create a lesion, and moving the fiber to a new location to form a subsequent lesion segment. A similar arrangement for an ablation tool is shown in U.S. patent publication No. 2002/0087151 published Jul. 4, 2002 (assigned to AFx, Inc.).
U.S. patent publication No. 2004/0102771 published May 27, 2004 (assigned to Estech, Inc.) describes a device to guide an ablation tool while maintaining contact between the heart and an ablation device. Other devices for either guiding an ablation element or for maintaining contact for between an ablation element and the heart are shown in U.S. Pat. No. 6,237,605 (assigned to Epicor, Inc.). The '605 patent describes using vacuum against an epicardium or an inflatable balloon against a pericardium to maintain ablation devices in a fixed position against the heart. U.S. Pat. Nos. 6,514,250 and 6,558,382 (both assigned to Medtronic, Inc.) describe suction to hold ablation elements against a heart.